Putting the new in nuclear

Ever since the beginning of the nuclear age, the peaceful use of atomic energy has revolved around the development and commercialization of water-cooled thermal nuclear reactors. Although those reactors have long generated a plentiful, carbon-neutral source of energy, they may soon no longer adequately meet America's needs. Argonne's expertise in designing a more resourceful and cleaner type of nuclear reactor has enabled enormous gains in the efficiency and safety of nuclear power.

For much of the second half of the 20th century, policymakers and scientists believed that newly harnessed atomic power held the key to solving America's energy needs for the indefinite future. The allure of new inexpensive, plentiful energy distracted many Americans from the complications of the new power source.

The 100 or so commercial nuclear reactors in operation in the United States have generated radioactive spent fuel that is currently stored at each reactor site but that now requires a long-term solution. Rising concern about the future of waste storage has prompted a call for a new generation of nuclear reactors that could dramatically reduce the number and size of repositories needed to safely store waste.

A fast reactor on a closed fuel cycle could – at least theoretically – use 90 percent of the energy available in uranium, reducing the amount of nuclear waste produced.

Fortunately, Argonne scientists have decades of expertise in creating a different type of reactor that could help to solve both this and other problems. Ever since the 1950s, Argonne's scientists have worked to develop fast reactors, which allow the recycling of many of the toxic isotopes that compose nuclear waste.

Fast reactors treat spent reactor fuel not as waste but as a rich source of recycled energy. This new kind of reactor system uses a fuel cycle that is better suited to recycling than that of today's light-water reactors.

"The characteristics of the current generation of light-water reactors eventually make the fuel unsuitable for recycling because of its long-term radiological toxicity after it’s discharged from the reactor," said Robert Hill, who manages Argonne's Nuclear Systems Analysis department.

"The current fuel cycle is incredibly wasteful," added Hussein Khalil, director of Argonne’s Nuclear Engineering Division. "Enrico Fermi recognized in the earliest days of nuclear power that the conventional uranium fuel cycle was extremely inefficient."

Because they permit the reprocessing of spent nuclear fuel, fast reactors can operate through what is known as the "closed fuel cycle," which dramatically increases the efficiency of uranium use and minimizes the discharge of plutonium and minor actinides as waste. According to Hill, a fast reactor on a closed fuel cycle could – at least theoretically – use 90 percent of the energy available in uranium.

Because fast reactors allow much of the uranium, plutonium and minor actinides to be fully recycled, their final waste products contain little of the extremely long-lived radioactive isotopes. "For every hundred or so repositories you'd need to house the waste from light-water reactors,' Khalil said, "you'd only need one for fast reactors that recycled spent fuel to produce an equivalent amount of energy."

The vast majority of commercial nuclear plants use water-cooled reactors that derive their energy from uranium-235, a fissile isotope that splits readily in the reactor core, releasing a great deal of nuclear energy. When a neutron traveling at a sufficient speed collides with uranium-235, the atom splits into two "fission products" – for example, isotopes of krypton and barium – releasing energy and more free neutrons to perpetuate the chain reaction.

However, the fuel used in these reactors is not entirely composed of uranium-235. The overwhelming majority – roughly 95 percent – of the uranium in the fuel is uranium-238 – a non-fissile form of the element. When a low-energy neutron in a light-water reactor hits uranium-238, it almost always either bounces right off or gets captured by the atom, transforming it into plutonium-239. Like uranium-235, plutonium-239 is highly fissile and thus can also generate tremendous amounts of atomic energy through fission.

As the chain reaction proceeds, a fraction of the neutrons that collide with plutonium-239 are themselves captured instead of causing fission, resulting in the creation of isotopes known as "higher actinides," which are radioactive and long-lived.

To prevent accidental exposure to the long-lived higher actinides and fission products, the spent fuel produced by today’s generation of light-water reactors must be isolated from the biosphere in remote areas that will remain geologically stable for tens or even hundreds of millennia. Just a few places in the country provide suitable locations for this kind of storage, and future demand for nuclear energy would only further complicate the selection of storage sites.

Instead of using water, fast reactors employ a coolant – typically liquid sodium – that does not “moderate,” or slow down, neutrons. The resulting “fast” neutrons are not as easily captured, and instead are more likely to fission most actinides.

In addition to discharging less hazardous material than conventional reactors, fast reactors use uranium much more efficiently. In a light water reactor, the initial fuel must be enriched to include a higher fraction of the fissile U-235; the large amount of U-238 discarded in the enrichment process is basically wasted.

Moreover, only about five percent of the enriched fuel can be consumed in the reactor if its physical integrity is to remain assured. "It's not like a car where you can take your tank all the way to empty," Khalil explained. "As you burn the uranium, the chain reaction eventually fizzles out and the fuel properties deteriorate. By the time you've gone through about five percent of your fuel, you just have to take the rest out and replace it with fresh fuel. All in all, you're using only about one percent of the uranium you took from the ground."

Conversely, with fast neutrons, fewer neutrons are captured by the actinides and fission products resulting in a more favorable neutron balance. The extra neutrons convert U-238 into fissile material, allowing it to be consumed almost entirely by fission and repeated fuel recycling.

Energy companies have not yet scaled up and fully commercialized fast reactor technology, not because of any inherent deficiency in fast reactor technology or operation, but because fresh uranium remains plentiful and cheap. "Right now, light-water reactors make more economic sense because the infrastructure for them is already established," Khalil said. "But the day may not be too far off when spent fuel accumulation and uranium scarcity become serious problems requiring other solutions."

The United States either currently faces, or may soon face, challenges in how it disposes of nuclear spent fuel and how efficiently it uses uranium. Today's commercial nuclear technologies do not offer viable long-term solutions to either of these challenges. Just as for other energy technologies, innovations in nuclear energy developed at Argonne help guarantee a safe, affordable and environmentally friendly component of our country’s energy supply.

"The vision, drive and knowledge of fast reactors and fuel recycling still all reside at Argonne," Hill said. "We have taken the lead in making sure that world recognizes their role in securing America's energy future."

These tiny branches, or "dendrites," of pure uranium form when engineers reprocess spent fuel from nuclear fast reactors.

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This article was published in the fall 2009 issue of Argonne Now, the lab's science magazine.

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